Apparatus for spraying cathodes



Aug. 7, 19 P. P. COPPOLA APPARATUS FOR SPRAYING CATHODES Filed July 27, 1959 INVENTOR fiATRICK P. COPPOLA,

VET TZSISW 3,048,146 APPARATUS FOR SPRAYHNG CATHODES Patrick P. Coppola, Fayetteville, N.Y., assignor to General Electric Company, a corporation of New York Filed July 27, 1959, Ser. No. 829,678 3 Claims. (Cl. 118-301) The present invention relates to improvements in thermionic emitters of the spray coated type and more specifically to an improved method and apparatus for making the same.

In an electron beam device such as a cathode ray tube it is most desirable to have an electron source of high and uniform emission density such as to provide an electron beam which is well defined, of uniform density, and easily and sharply focused. One factor affecting uniform emission density is the character of the surface of the emitter. With cathodes of the conventional spray coated type, irregularities in the surface of the spray coating exist which promote non-homogeneous field conditions. The strongest fields exist adjacent sharp projections on the emitter surface and cause localized hot spots of high emission density. Also such projections or peaks, in conjunction with the corresponding adjacent valleys, produce a series of lens-like distortions of the electric field which converge, diverge and deflect the escaping electrons, such actions having an adverse effect on both the definition and uniformity of density of the electron beam.

Various attempts have been made in the prior art to improve the surface smoothness of spray coated emitters but so far as I am aware none has proven entirely satisfactory. For example attempts have been made to produce smoother spray coatings by using a relatively wet spray. However this procedure has the disadvantage of promoting undesirably high coating densities and poor coating-to-coating substrate adherence. Another attempted solution involves the use of special spraying equipment for producing a so-called microspray having particles of an extremely finely divided nature. However such microspray equipment is costly, and the resulting coating does not provide the uniformity of smoothness of emitter surface desired.

A principal object of the present invention, therefore, is to provide an improved method and apparatus for making electron emitters of the spray coated type having a surface of improved smoothness such as to provide high and uniform emission density and minimize undesirable effects on electron beam formation.

Another object is to provide an improved method and apparatus for making spray coated emitters having surfaces of enhanced smoothness, yet involving a minimum of equipment and processing time to obtain a given quantity of emitters.

Still another object of the invention is to provide an improved method and apparatus for spray coating a substrate with emission material, which insures uniformly high quality without requiring special operator skills.

These and other objects of the present invention will be apparent from the following description taken in conjunction with the accompanying drawing wherein:

FIG. 1 is an isometric View of one form of spraying apparatus suitable for performing the improved spraying process of the present invention;

FIG. 2 is an enlarged fragmentary sectional View of a portion of the apparatus of FIG. 1;

FIG. 3 is another enlarged view of the portion of the structure of FIG. 1, looking in the direction indicated by the arrows 3-3 of FIG. 1;

FIG. 4 is a fragmentary sectional view of an emitter formed by conventional spray coating according to the prior art;

3,94%,i4fi Patented Aug. 7, 1962 ire FIG. 5 is a view similar to FIG. 4 showing an emitter made according to the present invention.

Briefly, according to the present invention I have discovered that the surfaces of spray coated emitters prepared by conventional spraying of emission coating material onto a suitable supporting substrate have a roughness which is due in large part to the intrinsic formation of comparatively large particle agglomerates of emission material at the peripheral portions of the locus of emission material issuing from the conventional spraying apparatus. I have further discovered that if such large particle agglomerates are prevented from reaching the emitter substrate surface by suitable masking, the end product is considerably improved both in smoothness and uniformity of surface texture. To prevent the large particle agglomerates from reaching the emitter surface I provide a novel masking or shielding arrangement between the spraying apparatus and the emitter surface, this shielding arrangement containing one or more aligned apertures which may be adjustable in size, and which are so dimensioned as to pass to the emitter surface only the center or fine-particle portion of the spray locus, while rejecting or baffling the larger coarser particles of the spray locus. The resultant spray coated emitter has a surface of exceptionally smooth and fine-grain quality, free of objectionable irregularities and the attendant undesirable hot spots and electric field distorting conditions above-mentioned.

Turning to the drawing, FIG. 1 shows one form of apparatus constructed according to the present invention for making spray coated emitters. The emitter spraying operation is carried out in a suitable box-like enclosure or housing 2 which has an access cover 4, and may be suitably ventilated as by filtered air through a ventilating system (not shown). Mounted on one end wall of the enclosure for rotation about a horizontal axis is a circular plate 6 to which a series of cathode holders 8 are secured in a circle concentric with the axis of rotation of the plate. Each cathode holder carries a plurality of cathode preassemblies iii, the details of each cathode preassembly being best shown in FIG. 2. The cathode preassembly consists of a metal cylinder or sleeve 12 closed at one end by a cap 14 and supported at its waist portion by an insulating disk 16 which is clamped between the opposing faces of top and bottom plates 18, 21) comprising the cathode holder. The top plate 18 of the cathode holder has an opening 22 through which emission material may be deposited on the exterior face of the cap 14, the cap serving as the supporting substrate for the emission material.

Within the enclosure is a spray gun 26 having a reservoir 28 for spray mix and means including a stirring motor 30 for keeping the spray mix homogenized. The spray mix may be any suitable conventional spray mix such as for example a suspension of one or more alkaline earth carbonates such as barium carbonate, strontium carbonate, and calcium carbonate, mixed with a suitable binder such as nitrocellulose or isobutyl methacrylates and an appropriate liquid vehicle such as ethyl alcohol, amyl acetate, ethyl acetate and the like. The spray gun is supplied through hose 32 with pressurized gas such as dry nitrogen which serves as the propellent for expelling the spray mix from the spray nozzle 34. The spray gun is pointed so that the axis of the spray locus defines the center of a desired target area along the path followed by the holders 8 when plate 6 rotates. To permit adjustment of the spacing of the spray gun from the rotatable plate, the gun is mounted on a support 36 which is movable on a bed or base and may be locked in place as by the clamp 38.

Disposed between the spray gun and the cathode holder-carrying plate is a shield or mask 40 in the path of the locus of spray material expelled by the gun. The mask is carried by a post 42 mounted on a plate 44 arranged to be adjustable in a direction parallel to the axis of the spray locus. The plane of the mask is generally normal to the axis of the spray locus.

The mask is provided with an aperture 46 which may be of any desired shape and may be adjustable in size, and which is centered substantially on the spray locus axis. Through the aperture the central portion of the locus of emission material expelled by the nozzle 34 may pass to the desired target area on the rotatable plate 6.

A second mask or shield for each emitter is provided by an inwardly extending lip 48 at the periphery of the opening 22 in each cathode holder 8 through which each cap 14 is exposed. The lip 48 may be adjustable in thickness, and defines a substantially straight walled passage through which spray particles pass to the emitter substrate to be coated. The lip also, if desired, may serve as a baflle preventing emission material from coating the cap 14 all the way out to its perimeter.

In the operation of the apparatus above described, the sprayer 26 may be operated either continuously or intermittently, and the plate 6 is rotated continuously so as to bring each cathode holder in turn into the target area covered by spray material passing through the aperture 46 in the mask. The cap 14 of each cathode preassembly in turn is thus coated with a layer of emission material only from that portion of the material in the spray locus, here shown for example as of conical shape, which passes through the aperture in the mask and past the adjacent lip. The fact that the exterior face of each cap 14 is recessed slightly below the external surface of the cathode holder by an amount equal to the thickness of the lip 48, enables the lip to serve as a second mask or shield preventing the cap from being reached by any particles of the emission material spray except those arriving substantially normal to the face of the cap 14, and effectively rejects undesired stray or random particles. The adjustment of the relative position of the plate, sprayer, and mask permits convenient adjustment of both the size of the area of the plate covered by sprayed emission material passing through the aperture as well as permitting adjustment of the size of the portion of the spray locuspassing through the aperture. Thus any desired portion of the coarser particles in the spray locus, which by reason of their greater size and weight inherently tend to be concentrated near the outer edges of the spray locus, may be rejected by the mask, and any desired degree of smoothness of final emission surface may be obtained up to the limit of fineness of the particles of emission material in the central portion of the spray locus.

FIG. 3 illustrates the comparative general appearance of the surface of the cap member and surrounding portion of a cathode holder coated by the portion of the spray locus passing through the mask aperture according to the present invention, and in sharp contrast thereto the portion of the surface of the mask surrounding the aperture. It will be apparent from FIG. 3 that the larger coarser particles tend to be concentratedin the outer portion of the spray locus and are intercepted by the mask and collected there while the smaller finer particles providing a smoother and more uniform emitter surface pass through the aperture in the mask and reach the emitter substrate.

In one satisfactory spraying arrangement according to the present invention a Devilbiss sprayer, Type EGA Series 502, with an E nozzle, an E needle and a number 395 cap was used. A jar of freshly rolled spray mix was attached to the gun and with the gun trigger closed propellent pressure was adjusted to 50 p.s.i. The speed of the rotatable plate was adjusted to about 20 revolutions per minute. The temperature in the housing was maintained at about 80 C. Using a generally conical spray locus, with a cone angle of about l emitter coatings of the order of .0025 inch to .0030 inch thickness were sprayed,

' spray mix.

a on substrates approximately A5 in diameter with a coating density of approximately 1.0 to 1.2 grams per cc. The. spray gun nozzle was spaced approximately from the shield with, the shield in turn approximately 7 from the emitter substrate face and with the shield having an opening of about 1 x 1. Approximately six passes of each substrate through the spray was utilized to build up the spray coating to the desired thickness.

Using the masking method and apparatus described I have found that the relative uniformity and smoothness of the emitter surface produced on'a cap member may be very substantially improved from a condition illustrated by the sectional view of FIG. 4 to the much more uniformly smooth surface illustrated in FIG. 5. The substantially increased smoothness and uniformity of cathodes made according to the invention, in comparison with prior art spray coated cathodes, is readily measurable. Heightvariation measurements of surfaces of a number of spray coated cathodes for cathode ray tubes, using a microscope with vertical fine-focus adjustment, indicate a height variation of about .00050 inch between peaks and valleys as shown in FIG. 5 of cathodes made according to the invention, as compared with a height variation of .00160 inch for conventionally sprayed cathodes of the same Thus it will be evident that the present invention provides a reduction in height variation or in crease in smoothness of the emitter surface of approximately three to one over the prior art. The practical benefits of such enhanced cathode surface smoothness and uniformity will be evident from the fact that in a number of cathode ray tubes provided with cathodes made according to the present invention, examination of the cathode images projected on the phosphor screen showed a substantial reduction in excited phosphor spot size at the screen. In some cases a spot size reduction as high as 25 percent was found, as compared with conventional spray coated cathodes of identical size and spray mix formulation operated under identical conditions.

Thus it will be evident that according to the present invention cathodes having surfaces of enhanced smoothness and uniformity of texture are provided. The smaller height variation between peaks and valleys of such cath odes minimizes strong local fields and hot spots and provides more uniform emission density throughout the area of the emitter surface with decreased electrical field distortion, and thereby produces electron beams which are more readily and'sharply focused for better definition and small and uniform excited phosphor spot size.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than those illustrative embodiments heretofore described. it is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for making thermionic emitters by spray coating a desired area of an emitter substrate member comprising a sprayer for expelling a slurry of particulate emission material in a conical spray locus having a reference axis, a holder movable to and from a spray position on the axis of said spray locus and no more than 6" from said spray orifice, means for supporting the emitter substrate member on said holder with said desired area to be coated arranged generally normal to the spray locus axis in said spray position, said holder having an opening provided with a peripheral radially extending lip masking from said sprayer the portion of said substrate area surrounding said desired area, the surface of said lip facing away from said sprayer being spaced from the plane of said desired area a distance slightly greater than the desired thickness of the spray coating to be formed on said desired area, and a spray shield between the sprayer and holder having an aperture substantially centered on the spray locus axis, said aperture being from 3 to 5 from said sprayer and having a size such as to permit only the central core portion of the spray to pass through said shield to said substrate mem ber, whereby the coarser particles of said spray are collected by said shield and prevented from landing on said substrate member.

2. Apparatus for making thermionic emitters by spray coating a desired area of an emitter substrate member comprising a sprayer for expelling a slurry of particulate emission material in a conical spray locus having a reference axis, a holder movable to and from a spray position on the axis of said spray locus and no more than 6" from said sprayer, means for supporting the emitter substrate member on said holder With said desired area to be coated arranged generally normal to the spray locus axis in said spray position, said holder having an opening provided with a peripheral radially extending lip masking from said sprayer the portion of said substrate area surrounding said desired area, the surface of said lip facing away from said sprayer being spaced from the plane of said desired area a distance slightly greater than the desired thickness of the spray coating to be formed on said desired area, and a spray shield between the sprayer and holder having an aperture substantially centered on the spray locus axis, said aperture being from 3" to 5" from said sprayer and having a transverse dimension of about 1" such as to permit only the central core portion of the spray to pass through said shield to said substrate member, whereby the coarser particles of said spray are collected by said shield and prevented from landing on said substrate member.

3. Apparatus for making thermionic emitters by spray coating a desired area of an emitter substrate member comprising a sprayer for expelling a slurry of particulate emission material in a conical spray locus having a reference axis, said spray locus having a cone angle of about 10 to 15, a holder movable to and from a spray position on the axis of said spray locus and no more than 6" from said sprayer, means for supporting the emitter substrate member on said holder with said desired area to be coated arranged generally normal to the spray locus axis in said spray position, said holder having an opening provided with a peripheral radially extending lip masking from said sprayer the portion of said substrate area surrounding said desired area, the surface of said lip facing away from said sprayer being spaced from the plane of said desired area a distance slightly greater than the desired thickness of the spray coating to be formed on said desired area, and a spray shield between the sprayer and holder having an aperture substantially centered on the spray locus axis, said aperture being from 3 to 5" from said sprayer and having a transverse dimension of about 1" such as to permit only the central core portion of the spray to pass through said shield to said substrate member, whereby the coarser particles of said spray are collected by said shield and prevented from landing on said substrate member.

References Cited in the file of this patent UNITED STATES PATENTS 2,440,084 Goda Apr. 20, 1948 2,676,841 Pohle Apr. 27, 1954 2,740,375 Diehl et al. Apr. 3, 1956 2,895,854 LaRo-cque July 21, 1959 FOREIGN PATENTS 660,924 Germany May 23, 1938 

